Atopic dermatitis (AD) skin displays epidermal permeability as well as antimicrobial deficiency, and is often colonized with virulent S. aureus. Ceramide (Cer) is both an essential permeability barrier lipid species and a bioeffector that alters multiple cellular functions in nucleated epidermal keratinocytes (KC). Importantly, a distal metabolite of Cer, sphingosine-1-phosphate (S1P) also modulates cellular functions, and levels of both Cer and S1P are reduced in AD. Pertinent to this proposal, AD epidermis shows a failure to upregulate a major epidermal cathelicidin antimicrobial peptide (CAMP)/LL-37 expression, which could account for S. aureus colonization and persistence in AD. We recently demonstrated that S1P stimulates CAMP expression not only through the well-characterized vitamin D receptor (VDR)-dependent mechanism, but also by a NF-?B-C/EBP-dependent pathway that is independent of VDR. Since we also have shown that VDR, but not NF- ?B -C/EBP transactivities, are suppressed in the epidermis under stressed conditions, both mechanisms are likely coordinately regulated to maintain CAMP expression under basal vs. stressed conditions, respectively. We further showed that S. aureus likely subverts S1P-induced upregulation of CAMP by secreting a neutral sphingomyelinase (SMase) (= -hemolysin) that preferentially diverts newly-generated Cer to glucosylCer (and sphingomyelin), which we hypothesize would simultaneously diminish S1P production, in parallel with a failure of sustained CAMP uperegulation. Cer and/or S1P could also upregulate ABCA12, a transporter protein that is likely required for CAMP secretion, via PPAR/ activation. In this proposal, we will investigate 1) the coordinate regulation of CAMP expression/secretion via VDR and NF- ?B -C/EBP mechanisms under basal vs. stressed conditions; 2) how a S. aureus virulence factor subverts the Cer->S1P signaling of CAMP production; and 3) novel, readily- translatable therapeutic approaches that both interdict the S. aureus virulence mechanism and enhance S1P signaling of CAMP production and/or secretion in normal and AD skin. PUBLIC HEALTH RELEVANCE: Atopic dermatitis (AD) is a common chronic inflammatory skin disease that often is complicated by chronic S. aureus infections that further exacerbate disease. We have identified a novel virulence mechanism that could explain why AD is almost always colonized by S. aureus. In parallel, AD patients have deficiencies in both the production of the innate immunity factor cathelicidin antimicrobial peptide (CAMP)/LL-37, and the levels of a newly-discovered stimulator of CAMP production, sphingosine-1-phosphate in epidermis. In this proposal, we wil investigate and develop novel therapies for AD by enhancing inate imunity through the stimulation of sphingosine-1-phosphate signal, and by interdiction of the novel S. aureus virulence mechanism.